The covering of a conductor for communications purposes is accomplished by pressure or by tubing extrusion. The conductor is moved through a core tube that is positioned in a cavity of a die and then through an exit orifice of the die. These are well known processes in which a plastic covering material engages the conductor either within the cavity of the die between the core tube and die orifice or is drawn down about the conductor after it leaves the die.
In the above-described pressure extrusion process, it is desirable to minimize the tension in elongated material such as a conductor which is being advanced along a path of travel through an extruder and covered. Undue tension in a metallic conductor such as copper, for example, causes the conductor to stretch which results in a change in its electrical properties. Should the conductor be a coated lightguide fiber, excessive tension may contribute to higher losses in the fiber.
It is conventional for a flow path of the plastic material between the outer surface of the core tube and the die wall to be inclined at an angle to the path of travel of the conductor. This design has been used to provide a transition from a relatively large annulus between the outer surface of the core tube and the die wall to the relatively small die orifice. Seemingly, this transition arrangement which is tapered would reduce the tension on the conductor. However, it has been found that substantial conductor tension still exists in a plastic extruder along the plastic-to-conductor contact length.
In order to reduce conductor tension in a pressure extrusion arrangement, the distance the conductor is moved through molten plastic after leaving the core tube must be reduced. However, once the core tube design is established in a typical die for extruding a covering onto a moving conductor, some dimensions of the die remain fixed and have not been changed. One example is the gum space which is the horizontal distance that the core tube can be moved toward the die orifice before it engages the die wall. As a result, the distance through which the conductor must travel after it leaves the core tube while it is in contact with the plastic material has been shortened by decreasing other dimensions. For example, in at least one arrangement, the distance between the gum space and the exit orifice of the die had been reduced.
The prior art includes U.S. Pat. No. 3,382,535 which issued on May 14, 1968 in the name of A. G. Ferrari. It was recognized therein that because viscous drag on a conductor is directly proportional to the length of a die, short dies permit the extrusion of plastic material on a conductor having a relatively low tensile strength. The above-identified patent discloses a relatively short die having an internal contour which varies from the exit to the entrance and which avoids melt fracture of the plastic material.
For tubing extrusion, wherein the plastic material does not contact a conductor until after the conductor leaves the core tube outside the die orifice, tension is not a problem. However, because an unsupported end of the core tube has a greater cantilevered length than in a pressure extruder, it is more susceptible to deflection. Unintended deflection of the end of the core tube which in a tubing extruder is positioned within a passageway connecting the die cavity with its exit orifice could affect adversely the concentricity of the conductor and its covering. Accordingly, concentricity becomes an important consideration in tubing extrusion.
What is needed and what seemingly is not provided by the prior art is an extrusion arrangement which minimizes conductor tension during pressure extruding and which optimizes control of concentricity in a tubing operation. The sought-after arrangement should be one which permits the retrofitting of existing extruders without undue expense.